Identifying Biological Signatures of N-Acetylcysteine for Non-Suicidal Self-Injury in Adolescents and Young Adults Non-suicidal self-injury (NSSI), the deliberate act of damaging one?s own tissues without suicidal intent, is a common problem in adolescents and young adults, and is associated with severe consequences including future suicide attempts. Treatments for this habitual, self-destructive behavior pattern are sorely limited. Identification of novel, biologically-informed treatments for NSSI in youth could improve health outcomes over the lifespan. N-acetylcysteine (NAC) is a natural product that is widely-available as an over-the-counter dietary supplement. It is also available as oral and intravenous (IV) prescription products, and may have promise as a novel treatment for NSSI in youth. In the first study of its kind, our open-label study showed reduced NSSI frequency (p=0.02) in female adolescents aged 13-21 after 8 weeks of oral NAC. In preparation for a definitive clinical NAC trial, a critical preliminary step is to clarify NAC?s biological signatures, or measures of the mechanisms underlying its clinical effects, which would provide the basis for biologically-informed design of optimized efficacy trials. This proposal will investigate possible biological signatures for NAC?s behavioral effects: (1) increased glutathione (GSH), the primary antioxidant in the brain; and/or (2) modulation of the glutamate (Glu)-cysteine antiporter, effectively decreasing excessive Glu transmission. Both of these mechanisms could alleviate stress-induced neurotoxicities in adolescents with NSSI, but neither mechanism has been evaluated in NSSI. Confirmation of NAC biological signatures is challenging due its complex pharmacokinetics (PK), posing uncertainty related to optimal dose. Oral NAC is primarily used in psychiatric studies because of its ease of use, but has low bioavailability (5-10%). Studies testing NAC for mental health indications have generally not paired clinical testing with PK or biological signature testing. To address knowledge gaps and advance development of NAC as a treatment for adolescent NSSI, we propose an R61/R33 project to investigate NAC?s biological signatures (changes in GSH and/or Glu) in young women with NSSI. The R61 will focus on identifying the optimal dose to achieve meaningful change in GSH and Glu during short-term NAC treatment. We will randomize 36 female adolescents and young adults aged 16-24 years with NSSI to 4 weeks of treatment with either oral NAC (3600mg or 5400mg per day) or to placebo. Go/NoGo criteria for advancing to R33 will be: ?Go? if any one of the following occurs: (a) ? 50% increase in blood GSH redox ratio; (b) 5% increase in brain GSH concentrations; (c) 5% decrease in brain Glu concentrations. Dose and for the R33 will be selected based on the biological changes and the tolerability observed in the R61. The R33 phase will seek to replicate the biological signature findings in an 8-week trial using chronic dosing in a new sample, and examine the relationships among biological signatures, NAC PK, and clinical response.